Serve MCP and hooks with a secret through the portal tunnel

The connector now serves /mcp, its OAuth documents and /hooks/ besides
/api/v1/, refuses any dot segment (httpx resolves them, which carried
/api/v1/../../mcp onto the MCP mount), addresses calls to the hub's host,
and bridges served sockets by calling the app with a websocket scope.
A hook without a secret answers the tunnel with the 404 (or 1008) of a
wrong secret. Requests through the hub get the hub's issuer, resource and
consent page in the OAuth metadata and the 401, and the MCP host check
admits the hub while enrolled.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

docs/reference/node-types.md

@@ -80,6 +80,8 @@ publishes nothing.
 
     The canvas flags this as an advisory issue rather than stopping the flow,
     because a webhook on a private network is a legitimate thing to want.
+    Through the [portal](../interface/portal.md#hooks-and-agents-through-the-portal)
+    a webhook or served socket answers only with a secret.
 
 ### WebSocket